A spark plug used in a combustion device such as an internal combustion engine includes, for example, a center electrode that extends in the axial direction, an insulating body that is provided in the outer periphery of the center electrode, a cylindrical metal shell that is assembled to the outside of the insulating body, and a ground electrode that has a base end portion bonded to the leading end portion of the metal shell. The ground electrode is curved at the substantially middle portion thereof so that its leading end portion faces the leading end portion of the center electrode. Accordingly, a spark discharge gap is formed between the leading end portion of the center electrode and the leading end portion of the ground electrode.
Further, in recent years, a technology has been introduced which bonds a noble metal chip to a portion provided with the spark discharge gap in the leading end portion of the ground electrode in order to improve wear resistance. As a technology of bonding the noble metal chip, for example, a method is suggested which forms a fusion portion used to fuse a noble metal chip and a ground electrode to each other by laser welding, and bonds the noble metal chip and the ground electrode to each other through the fusion portion (for example, Patent document 1 and the like).
However, the fusion portion has wear resistance worse than that of the noble metal chip. Further, since a minute uneven portion may be formed on the surface of the fusion portion, a spark discharge is generated between the center electrode and the uneven portion having a comparatively large electrical field strength, so that there is a concern in that ignition performance may be degraded. Accordingly, from the viewpoint of preventing degradation of the ignition performance or the wear resistance, it is desirable that the fusion portion is exposed to the side of the spark discharge gap as little as possible. Therefore, a technology is suggested which forms a concave portion in a ground electrode, disposes a noble metal chip in the concave portion to be buried therein, and emits a laser beam from the side surface of the ground electrode toward the buried portion of the noble metal chip, so that the fusion portion is suppressed from being exposed to the side of the spark discharge gap (for example, refer to Patent document 2 and the like).